A flue gas or a diesel engine exhaust gas contains particulate matter (PM) (e.g., soot) that causes air pollution. A filter (diesel particulate filter (DPF)) formed of a ceramic or the like has been widely used to remove them. A DPF formed of a ceramic can be used for a long period of time, but may suffer defects (e.g., cracks) due to thermal deterioration or the like so that a small amount of particulate matter may leak from the DPF. It is very important to immediately detect such defects and abnormalities of apparatuses from the viewpoint of preventing air pollution. The amount of particulate matter discharged to the outside may increase when the amount of particulate matter contained in exhaust gas increases due to malfunction of a diesel engine or the like. In this case, it is important to detect the particulate matter contained in the exhaust gas to detect malfunction of a diesel engine or the like.
As a method of detecting particulate matter contained in exhaust gas, a particulate matter detection device may be provided on the downstream side of a DPF (see Patent Documents 1 to 5, for example).    Patent Document 1: JP-A-60-123761    Patent Document 2: JP-A-2006-503270    Patent Document 3: JP-B-6-39937    Patent Document 4: JP-A-2006-46281    Patent Document 5: JP-A-2005-91043